Issue
Leads are not syncing from SFDC to Marketo after the sync user has gained access to them.
Solution
Make a change to the lead records to queue them to sync.
SFDC has permissions that allow specific users to access some leads but not others. Your Marketo SFDC sync user is a user that can have this setting applied. The sync user can also have that restriction removed to allow access to those leads.
When Marketo checks Salesforce for people to update, it is looking for leads who have had record updates since the last sync. Changing visibility rules in SFDC to allow the sync user access to those leads does not update the records themselves, only the accessibility to those leads by the sync user. This means that if the sync user couldn't access the leads before but now can, they still won't sync to Marketo until they have a field update.
You can change a field value, but another option is to run the Data Loader in SFDC for those affected leads. That will create a new modification indicator, which will trigger Marketo to sync those records down.
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